The invention relates generally to a process and arrangement for cooling fluids.
The invention may find particular application in so-called "Base-Load" arrangements or apparatus which are used for the liquefaction of natural gas, such liquefaction being desirable, for instance, when the natural gas is to be transported overseas. The invention may also find utility in other fields of application such as, for example, the liquefaction of helium.
In the construction of Base-Load apparatus for liquefying natural gas, the design considerations are directed to increasing the capacity of the gas trains in order that an apparatus which is optimum with regard to economic factors may be obtained. The capacity of the gas trains where a single cooling circuit, having a single, if necessary, multiple-step compressor, is utilized is, however, limited by the capacity of the compressor. As a result, it is already known to additionally utilize a precooling circuit so as to divide the work of compression between two compressors. A process using such a precooling circuit is disclosed, for example, in the French Pat. Nos. 2,045,058 and 2,076,029.
Also known is a process for cooling fluids wherein an incorporated cascade circuit and a precooling circuit are used and wherein the cooling fluid in the incorporated circuit consists of a mixture which is compressed and cooled with a cooling medium. The fluid in the incorporated circuit is in the form of superheated vapor subsequent to cooling and this superheated vapor is then partially condensed. The pressure on the resulting condensate is at least partly relieved and the condensate is vaporized, heated and returned to the compressor. The precooling fluid in the precooling circuit is also compressed and then cooled with a cooling medium and condensed. The pressure on the resulting condensate is at least partly relieved and the condensate is vaporized, heated and returned to the compressor. The precooling fluid thus obtained by evaporation and heating serves for cooling and condensing the cooling fluid in the incorporated circuit when it undergoes partial condensation.
A single component precooling fluid is utilized in the precooling circuit and this single component precooling fluid is not supercooled. It has been found that because of this it is not possible to obtain optimum utilization of the precooling fluid in the precooling circuit.